Further to my recent post I updated some of my maps following some discussions I’ve had. I’ve added some climate indices, namely Growing Degree Days and the Huglin Heliothermic Index. I could add in other standard ones too but have decided not to since it’s my view that since these indices are measures calibrated on less marginal viticultural climates, and their use in a marginal climate should be done with caution. Unsurprisingly the resultant mapped indices are highly correlated with the temperature map. The slight exception is that the Huglin Index map shows more high-value areas inland than the GDD or temperature maps do. This is explained by a slight continentality effect because it uses the average of maximum temperature and mean temperature as its driver, rather than just mean temperature.

2 Responses to vitiMapUK – updated maps with climate index maps

Personally I’m not wild about taking a “growing season” approach in our latitudes & essentially maritime climate. If you consider a base GDD temp of 10 C (which is a broad approach anyway, varies by cv), or look at temperatures outside those months, or look at budburst dates – any of those show that the vines are kicking in from a lot earlier. Sap was flowing in Plumpton main patch in mid-February 2009, just for example… Off the back of this, I’m also in favour of looking at growing degree hours rather than days, coz that makes a BIG difference. NB: I know I’m ploughing a lonely furrow on this one.

I agree that applying any viticultural index or rules-of-thumb to the UK should only be done with caution. These tools, whilst useful, are crude and are typically developed by calibrating input to output data (e.g. temperature summation to yield) in specific locations which can be different to the UK in many other ways. I also agree that it makes sense to use more detailed data for temperature summation (e.g. degree hours) since it makes intuitive sense that such a measure is closer to what’s really going on in the vines. Of course it’s still just a calibrated metric which should be treated with caution. My hope was to be able to analyse historic yield data from many UK producers and look at correlation of historic vintage variation to weather. By doing this it would be possible to create viticultural weather-indices specific to the UK. Clearly such index creation would be vulnerable to the complexities of the real world muddying the picture, but I would hope that with enough data this might even out, making the indices valid. Sadly it’s been tricky to get hold of quality data, and many producers are also protective of such numbers despite my protestations of confidentiality. However, I do have 10 years of data for one group of vineyards and have been able to cook up a very simple climate/yield model with a correlation over 80%. This is clearly a spectacular number, but one which should be treated with enormous scepticism until many more producers can be analysed and until any such model is “blind” tested on real world data as we go through time. It’s important to remain sceptical on this since although the model has a very intuitively appealing parameterisation, the details were chosen to me with an eye on getting a high weather/yield correlation, hence the model is “fitted” to describe history and its ability to predict the future is utterly untested. For you information my climate/yield model is currently saying that the 2011 harvest will be +12% on long term average, after taking into account increases and decreases for maturing and ageing vines. And yes, the model is based only on monthly data, the drawbacks of which you are clearly aware.

Anyway, a simpler answer is to say that the maps I created give a good picture of the relative climate in different areas of the country, and for that I find them of interest.